Embodiments of the present invention relate to a loudspeaker comprising a pressure compensation element.
Loudspeakers serve to convert an electric alternating signal, for example a sinusoidal signal, to sound or airborne sound. As is depicted in FIG. 3a, a loudspeaker 5 typically comprises a housing 10 having an enclosed volume V14 (e.g. 5 I-10 I), and one or more sound transducers 12. The sound transducers 12, which are frequently configured as piston vibrators, typically include a membrane (diaphragm) 12a and a voice coil 12b as the drive.
The voice coil 12b is configured to cause, upon application of an alternating signal, the freely supported membrane 12a to vibrate. This results in an excursion of the membrane 12a or of parts thereof, both into the housing 10 and from the housing 10, so that the gas volume V14 enclosed by the membrane 12a and the loudspeaker housing 10 is varied inside the housing 10. Starting from a closed housing 10, a change in pressure takes place inside the housing 10 because of the variation of the gas volume V14 since said housing 10 is spatially separate from the external volume, and since, therefore, different pressure conditions may arise on the inside or outside of the membrane 12a. It shall be noted at this point that without said separation, pressure compensation processes may occur which are also referred to as acoustic short circuits and which result in clearly reduced sound generation.
Since the gas volume V14 inside the housing 10 is compressed when the membrane 12a moves into the housing 10, the volume counteracts the movement of the membrane like a mechanical spring. The reason for this is that the compression process upon the membrane 12a (along with the drive 12b) moving into the housing results in the formation of an excess pressure in the gas volume V14, which leads to a spring force Ff of the gas volume V14. This spring force counteracts the movement of the membrane during the compression process of V14. It shall be noted that by analogy herewith, a spring force −Ff results, when the membrane 12a moves out of the housing 10, due to an arising negative pressure in the gas volume V14. The spring forces Ff and −Ff are proportional to the air spring rigidity s14, which is dependent on the area of the membrane 12a and on the size of the gas volume V14 within the loudspeaker housing 10. Thus, the air spring rigidity s14 is proportional to 1/V14. The frequency response of a loudspeaker 5 and, thus, the sound quality are influenced substantially by the air spring rigidity s14. A resulting frequency response for the loudspeaker 5 is depicted in FIG. 3b. 
FIG. 3b outlines the of the sound pressure level p(f) across the frequency f of an idealized loudspeaker 5. In addition, the diagram depicts the impedance curve Z(f) across the frequency f. As can be seen from the amplitude frequency response p(f), the loudspeaker 5 has a lower cut-off frequency fG which is defined, for example, by the −6 dB point in the frequency response and amounts to 40 Hz, for example. From the impedance curve Z(f), the resonant frequency fR may be determined, which is located at the peak, or maximum, thereof and here amounts to 60 Hz, for example.
FIG. 3c shows a further loudspeaker 5′ comprising a housing 10′ and the sound transducer 12. The housing 10′ comprises (as compared to the housing 10) a reduced gas volume V14′ (V14′<V14) of, e.g., 0.5 liters or 1 liter. The reduced volume V14′ results, in accordance with the above mentioned relationship s14′˜1/V14′, in increased spring rigidity s14′ for the enclosed air volume V14 (s14′>s14). Also, the size of the spring force Ff to be overcome is crucially dependent upon the amount, in percent, by which the enclosed gas volume V14′ is reduced or increased by the movement of the membrane into the housing 10′ or out of same, respectively. The larger the percentage change in volume, the larger the force Fm or −Fm that may be exerted in order to overcome the air spring. This results in that given the same size of the membrane 12a and the same deflection thereof, in a smaller housing 10′ or gas volume V14′, a larger force Fm or −Fm may be used, due to the increased air spring rigidity s14′, in order to overcome the air spring than for a larger housing 10 or gas volume V14. Since, as was explained above, the transmission characteristic is dependent on the spring rigidity s14′, a change or reduction in size of the housing 10′ results, when the same sound transducer 12 (chassis) is used, in a change in the frequency band, as is depicted in FIG. 3d. It shall be noted that the overall rigidity of the chassis 12 is composed of the air spring rigidity s and the rigidity of the membrane suspension. Consequently, the air spring rigidity s is important in particular when it is no longer negligible as compared to the rigidity of the suspension of the membrane, e.g. with small loudspeaker housings 10′ (having a small gas volume V14′).
FIG. 3d shows a diagram of the impedance curve Z(f)′ and the amplitude frequency response p(f)′ for the loudspeaker 5′ of FIG. 3c (plotted across the frequency f). As can be seen from the impedance curve Z(f)′, the resonant frequency fR′ is shifted upward due to the smaller housing 10′ and is now at 100 Hz, for example. Likewise, the lower cut-off frequency fG′ is increased (e.g. to 80 Hz), as may be seen from the amplitude frequency response p(f)′. In addition, in the amplitude frequency response p(f)′, a resonance step-up is formed in the range of the resonant frequency fR′, which has a negative impact on the linearity of the frequency response p(f)′.
In many cases of application, there is the desire, in particular for optical reasons, to have a loudspeaker housing 10 that is as small as possible and that accommodates potential electronics for controlling the sound transducer 12 (e.g. frequency separator, amplifier). Even when the size of the membrane 12a remains unchanged, the size of the housing 10 or 10′ may be varied within a limited range. However, since the size of the housing 10 or 10′ has a direct impact on the linearity of the frequency response p(f) or p(f)′ and the transmission range, in particular, the lower transmission range (cf. lower cut-off frequency fG or fG′), as was explained above, there is a conflict between the size of the loudspeaker 5 or 5′ and the sound quality.